A common mode choke is also an inductor, and it works to filter specific frequencies. In particular they are used to filter the high frequencies while allowing low frequencies to pass through this passive electronic component.
The common mode choke is considered passive because it doesn’t create a particular action to function. It isn’t a switch, but it can effectively filter the frequencies, allowing only specific low frequencies to pass through.
It is designed to be very simple and efficient. All of these designs will have a two wire winding and one core, which will have a toroidal shape. They can be large or small, and they will have a different number of windings based on the specific type of inductor and the requirements of the electronic device.
What is Common Mode?
Common mode is different than normal mode. Normal mode is also called differential mode current and it used in many types of circuits. In Normal Mode Current from the AC mains travels through the AC line and out of the AC neutral. This results in noise if the transmission on the line side is not offset by the return on the neutral side.
Common mode chokes work by both balancing the system as well as cancelling the common mode noise, or that noise that is present in the AC mains. It is called common because it is found on both the neutral and the mains.
The Choke
The common mode choke is made up of a magnetic or ferromagnetic core around which two separate windings are placed. Both of the windings are identical to each other to create symmetric in the system, but they are wound in opposite directions. This creates a mirror image of each side, with one side, the primary winding, bringing in current from the AC mains and the secondary coil providing the balance or cancelling out of the flux generated by the primary coil.
The result of using a common mode choke is that the normal mode currents pass through the passive inductor while the common noise mode is actually eliminated by the same flux.
The key in finding the right common mode choke is to choose the right level of suppression required for the common mode noise. To ensure the maximum efficiency it will be important to ensure the lowest frequency for common mode noise suppression is used in selecting the right choke for the design.
